The invention proceeds from a process for regulation of resistance welding operations in which by measuring the current strength of the welding current and the voltage at the welding electrodes, the dynamic resistance or resistance index of the workpiece to be welded is determined during the welding operation, stored and evaluated in comparison with an empirically determined correcting or command variable.
In process and apparatuses with measurement of the dynamic resistance during the welding operation, the resistance of the workpiece is repeatedly determined and related to the quality of the joint to be expected. Due to the low measuring technology expenditure, only physical quantities (voltage and current) being measured, which drop in spot welding, this process is preferably used for spot welding control. In all other known control methods of this type particularly distinguished quantities are determined in a respective half or whole welding period, for example the resistance at the instant of the current maximum or a resistance-similar quantity which results from the quotient of the integrated voltage with respect to the integrated current (MIACHY method). The variation of these distinguished quantities is determined over the entire welding time and when a switch-off criterion is reached the welding is terminated. The switch-off criteria are determined empirically.
These known processes use for the regulation the welding time or the number of welding periods per welding spot as the controlling variable. The energy introduced into the weld spot, which can be varied by the phase angle of the primary voltage, can be employed as additional controlling variable. DE-OS No. 3,301,039 discloses a process in the work integral introduced at the welding point is determined and within each mains half period the energy supply to the welding point is switched off after reaching a work integral corresponding to a desired value for the respective mains half period. To control the electrical work, thyristors which can be switched off are preferably employed.
In the known process due to inadequate information density or information evaluation, an advance recognition of a disturbance in the welding process is not possible.